NEW MAPPING RESULTS FROM THE UNCOMPAHGRE PLATEAU NEAR UNAWEEP CANYON

The Uncompahgre Plateau of the northeastern-most Colorado Plateau is a northwest-southeast trending fault block about 45 km wide and 95 km long. The northwestern edge is a well-described faulted monocline, but the southwestern edge and adjacent regions in the vicinity of Unaweep Canyon are less well characterized, owing to a lack of detailed mapping. The Plateau is cored by Precambrian basement, and capped by Mesozoic strata, except where breached by Unaweep Canyon.

As part of the USGS EDMAP program, we conducted new mapping of several 7.5’ quadrangles in the region of the Uncompahgre Plateau encompassing western Unaweep Canyon, an enigmatic landform that bisects the Plateau, incising through Mesozoic overburden and into Precambrian basement of the Plateau core. Among other goals, detailed mapping of the region around the canyon will enable a better understanding of the structure, and thus deformation of the Uncompahgre Plateau, with potential bearing on the origin of Unaweep Canyon, which has been hypothesized to have originated during late Paleozoic time. If the inner gorge of Unaweep Canyon dates from the late Paleozoic, then palinspastic restoration of the nonconformity surface is required to reconstruct the original profiles of the paleo-canyon.

We will present new mapping results that (1) place improved constraints on the preserved Mesozoic strata atop the plateau, and the reconstructed Mesozoic burial, (2) reveal additional, previously unrecognized faults associated with the Ute Creek Graben region along the southwestern margin of the Plateau, and (3) reveal the structure on the nonconformity surface both along and across the strike of Unaweep Canyon and the plateau crest. Additional data analysis should enable assessment of possible post-Paleozoic structural influences on the nonconformity surface along profiles extending transverse and longitudinal with respect to Unaweep Canyon, shedding further light on the possibility of a Paleozoic age for the inner gorge of the canyon.